Early in post-natal development heart muscle cell division is arrested and subsequent growth results from hypertrophic increases in cardiac myocyte size. Although hypertrophic growth ceases in the mature heart, many cardiac-related disorders, such as hypertension, provoke its reinitiation. This renewed growth is initially compensatory but eventually leads to a decompensatory reduction of cardiac muscle mass due to myocyte apoptosis, decreased cardiac function and, ultimately, to heart failure. Our long-term objective is to understand the signal transduction mechanisms that regulate cardiac growth and apoptosis. This proposal addresses the hypothesis that the p38 mitogen activated protein kinases (MAPK) play pivotal roles in determining the balance between cardiac myocyte growth and apoptosis, and that the relative activity states of p38 isoforms, e.g. p38alpha and p38beta, serves as a crucial determinant of myocyte fate. To address the mechanism by which p38 isoforms can be differentially activated in the heart, we propose the following Specific Aims: 1) To identify members of proposed p38 signaling complexes in cardiac myocytes. Using Western- and Northern analyses, as well as yeast two-hybrid screening, proteins in the heart that comprise complexes that direct signals toward growth or apoptosis, including hypothetical scaffold proteins, will be identified and, in the case of newly-discovered molecules, their structures and properties determined. 2) To characterize the cellular functions of p38 signaling complex members. Here, we will use a novel antisense oligonucleotide approach and overexpression of key signaling proteins to selectively perturb levels of signaling participants in cultured cardiac myocytes. Effects on the gene expression and morphological characteristics associated with cardiac hypertrophy and apoptosis will be assessed. 3) To elucidate the effects of manipulating the levels of p38 signaling complex members on heart structure and performance in vivo using cardiac-targeted transgenic mouse models. These studies employ novel combinations of powerful molecular approaches to unravel the roles of the recently-discovered p38 MAP kinases in cardiac myocyte growth. We anticipate that the results will provide new information that is required to move the field forward in the search for gene therapy targets aimed at solving problems related to the unusual hypertrohic growth program exhibited by cardiac myocytes.